Many years ago vehicles were provided with bumpers which bend elastically and restore under low velocity impact loads, the bumpers usually comprising a single spring-like plate. These bumpers had limited utility.
Then came a long period when rigid bumpers were used, largely ornamental, and easily deformed under relatively light impact loads. Such bumpers afforded but little protection to the vehicle or its occupants and were expensive to replace. There were proposals to use pneumatic beams and various other forms of bumpers.
More recently there have been requirements that bumpers take low velocity impact loads, at least up to about five miles per hour, to protect the vehicle and its occupants while preserving the bumper system. The bumpers now commonly supplied to meet this requirement comprise a rigid bumper bar of hollow section backed by shock abosrbers, as of the reciprocable fluid piston-cylinder type. The bumper bar may have a maximum bending deformation before permanent yield of about 1/8 inch when a load of 20,000 pounds is applied. The shock abosrbers may provide a movement of about 2 inches at impact loads averaging about 20,000 pounds over this distance.